1. Field of the Invention
The present invention relates to a silicon nitride sintered body used for wear-resistant parts such as cutting inserts, bearings, and balls; a method for manufacturing the silicon nitride sintered body; and a cutting insert formed of the silicon nitride sintered body.
2. Background Art
Conventionally, silicon nitride sintered bodies predominantly containing silicon nitride and having an excellent strength have been used for cutting inserts, wear-resistant parts, etc.
In recent years, for the case where silicon nitride is used for cutting inserts, there has been proposed a technique of decreasing the amount of a sintering aid (an oxide) which is added to the essential component, in order to enhance wear resistance of the sintered body (See PCT Kohyo Publication 8-503664 or U.S. Pat. No. 5,525,134). As disclosed therein, a sintered body contains at least about 85 volume percent (vol. %) silicon nitride and less than 5 vol. % intergranular phase. The ceramic has greater than 0.2 weight percent (wgt. %) magnesia, greater than 0.2 wgt. % yttria wherein the sum of magnesia and yttria is less than 5 wgt. %. The ceramic also includes between 1.3 wgt. % to 2.2 wgt. % oxygen.
However, with the conventional cutting insert, there is a drawback that an extreme decrease in the amount of the sintering aid to be added renders sinterability lower than expected and decreases wear resistance and chipping resistance of the sintered body.